A thin-walled panel may be stiffened by attachment of a plurality of stiffeners, which divide the panel into a plurality of smaller and more stable sub-panels. Stiffener reinforced panels are encountered in many weight optimised structures, such as aircraft structures. For example, an aircraft wing box typically includes upper and lower covers, which comprise a skin (panel) stiffened with reinforcing longitudinal stringers (stiffeners).
Stiffeners have to be terminated in certain areas due to conflict with other structural components. In the case of an aircraft wing box, the stiffeners typically have to be terminated at, e.g. the wing root, the wing tip, access holes etc. At the stiffener termination, stress supported by the stiffener is transferred into the panel. Stiffener terminations are therefore often responsible for areas of local stress concentration. To reduce the stress concentration, stiffeners are typically designed to have a, so called, “run-out” region at the termination to transition the load from the stiffener into the panel in such a way as to minimise stress peaks.
With the move to greater use of composite materials, such as carbon fibre-reinforced plastic (CFRP), in weight optimised structures the structural performance at the stiffener run-out is of particular interest.
For a composite aircraft wing box cover, the stringers are typically bonded (co-cured, co-bonded, or secondary bonded) to the skin. The stringer run-outs can suffer cracking when loaded due to the natural offset between the stringer and the skin. The offset causes out of plane bending, which results in tensile through-thickness loads that can cause peeling at the (tip) end of the stringer foot run-out where it attaches to the skin.
This problem has traditionally been solved by ensuring low deformation (low strain) in the structure under load. A solution which enables the structure to work at higher strains without issue would enable weight saving. U.S. 2005/0211846 A1 describes a stringer run-out having a gradual reduction in the stringer height at the run out, and WO 2008/132498 A1 describes fastening a stringer run-out to a panel to enhance performance.